what forms the channel in a MOS

[inquisitive]

Say V(S)=V(D)=0, VGS>VTH.
My doubt is : What forms the channel? Are the electrons that form the inversion layer generated by EHP in the substrate? or they are injected from the source?

I didnt find the exact answer in any text book. Please help.

 

[Willt]

Provided that Vgs - Vth > 0, a channel is formed from the electrons in substrate which is connected to the ground for NMOS.
When there is potential difference between drain and source, current flows through the transistor.

 

[A_U_J]

Thats a good question.
Well,I think when VDS=0 and VGS>0, then channel in an NMOS transistor is due to the e-s in the substrate.(minority carriers in the substrate.Hence off current of a transistor is pretty low,similar to that of a diode)
However when VDS>0,then we have additional e-s from source contributing to current.Additonally we have a drift mechanism which accelerate holes from source to reach drain.

So , conclusion:
1)VDS<0 and VGS>0, e-s are from p-sub in NMOST
2)VDS>0 and VGS>0. e-s are from both n-source and p-sub in NMOT.

Hope this helps.
PS: I do not have any concrete proof for this but I have discussed with other experienced guys and they feel above explanation is correct.

 

[swarnavenu]

Hi,

When VGS is positive, the positive voltage on the gate attracts the electrons(minority carriers) from the substrate.Also because source and drain have plenty of electrons,and we have(Cgs and Cgd overlap caps), some of these electrons contribute to the channel formation.

 

[quaternion]

For NMOS:

Increasing VGS will attract electrons (subs. minority carriers)- by the electric field across the oxide- near the oxide surface , when the electrons concentration at the oxide si interface = the holes concentration at the substrate it is said that that this applied VGS = Vth.(this is Vth defin. independent of VDS)

I think that what A_U_J said is right.

But what about the LDD formation which is done to minmize carrier injection from the source/drain regions ?

 

[s_babayan]

dear friend, I suggest you to study this book:

1/analysis and design of analog integrated circuits by Gray